Conventional printed circuit boards are formed by utilizing cloth, particularly of fiberglass, which is then impregnated with a resin such as epoxy, and partially cured to a "B" stage condition. Copper foil treated on one side to effect good bonding may then be placed on one or two sides of the pre-impregnated "B" stage material. The thusly resulting assembly is placed between steel platens with a suitable mold release or a release film between the platens and the assembly is cured under heat and pressure to form a laminate, copper clad on one or two sides.
The copper clad or possibly unclad laminates are subsequently used for the production of printed circuit boards by a subtractive or additive process. Printed circuit boards are used in the electronics industry for circuitry. In general, holes have to be formed in the structure for the insertion of component leads (i.e., for resistors, capacitors, integrated circuit chips, transistors, etc.) or for the interconnection of circuits from one surface to one or more other surfaces.
The forming of holes (usually by drilling) in laminates reinforced with fiberglass or other hard reinforcing fiber material is difficult, expensive and creates problems which necessitate additional operations. The drilling runs into problems such as epoxy smear where the drill heats up, due to friction, to a temperature higher than the glass transition temperature of the plastic matrix and causes this plastic to smear over the edges of the copper. This smear must be removed by etch back to re-expose the copper to insure a proper connection with the thru-hole plated copper.
In researching prior art patents for purposes of examining the uniqueness of the present invention, U.S. Pat. No. 3,537,937 dated Nov. 3, 1970 has been encountered. Therein is disclosed an arrangement in accordance with which longitudinally arranged filaments are applied to opposite surfaces of an endless flat metallic band and additional filamentary rovings are helically wound thereover. The filamentary structure is resin impregnated before the band and structure pass through a resin curing apparatus wherein the resin is partially cured. Upon emerging from the curing apparatus, the resin and filamentary material along the edges of the band is removed and separate top and bottom filament wound resin sheets of continuous length are removed from the band and wound on spools.
This prior art could be suited for producing structural panels for mechanical loads. It does not contain teaching on structurally stable and balanced and flat thin sheets.
The method of U.S. Pat. No. 3,537,937 cannot produce layers wherein the filament sections are perfectly perpendicular to one another.
It is, accordingly, an object of the present invention to provide an improved laminate adapted for use in the manufacture of printed circuits and an improved method for making the same.
It is a further object of this invention to provide an improved laminate adapted to permit precise positioning of components.
It is yet another object of this invention to provide a filament-wound resin-bonded structure capable of being punched or at least drilled without the drills touching the reinforcing fibers. Additional objects of the invention are set forth below.
It is a further object of the invention to provide for the design and control of the thickness of laminates, and the reinforcement-content-to-resin-content ratio to values and tolerances not attainable with the previously known state of the art. The number of variables in the previously known process (such as weight of fabric, resin content, age and rheology of the prepreg, variation in heat history in all directions X, Y and Z of the book, i.e., many layers of laminates and separator plates between platens in a press) lead to wide variations in temperature versus time between laminates and even in parts of the same laminates. In the present invention, the laminate is molded to the exact thickness required with precise repeatability. The reinforcement can moreover be applied in situ in the exact amount the design calls for, even compensating for its yield variation, i.e., yards per pound. The resin matrix takes up the rest of the space and there are no voids. All this leads to latitude of design and tight control of wall thickness and reinforcement-to-resin ratio.